Deposition of a film on the surface of a semiconductor wafer is a common step in semiconductor processing. The process of depositing layers on a semiconductor wafer (or substrate) usually involves placing the substrate within a processing chamber and holding the wafer within a stream of a reactant gas flowing across the surface of a wafer. Usually, heat is applied to drive the chemical reaction of the gases in the chamber and to heat the surface of the wafer on which the film is deposited. The processing chamber is typically heated by external lamps which pass infra-red radiation into the processing chamber through a quartz window that is transparent to the infra-red radiation.
Referring now to FIG. 1, there is shown a multiple-chamber integrated process system 100 including an enclosed main frame or housing 102 having sidewalls that define an enclosed vacuum transfer chamber 104.
A number of individual processing chambers 106a-f are mounted one each on an associated sidewall of the transfer chamber 104. Two load lock cassette elevators 108a and 108b are adapted for vertically stacking a multiplicity of cassettes which in turn hold wafers 110 horizontally. The load lock cassette elevator assemblies 108a and 108b selectively position each cassette directly opposite and aligned with a transfer chamber entrance slit or opening 112a and 112b, respectively. Each cassette holds multiple wafers. Wafers 110 are held within the cassette by a set of support structures 111 having a diameter that is slightly larger than the diameter of the wafers being housed.
Processing chambers 106a-f and the associated main frame side walls also have communicating slits 114a-f, respectively, which are similar to the load lock entrance slits 112a and 112b. Doors or slit valves (not shown) are provided for sealing the access slits.
A robotic wafer transfer system 120 is mounted within transfer chamber 104 for transferring wafers 110 between load locks 108a and 108b and the individual processing chambers 106a-f. Robot assembly 120 includes a blade 122 and a driver (not shown) that imparts both rotational and reciprocating movement to blade 122 for affecting the desired cassette-to-chamber, chamber-to-chamber and chamber-to-cassette wafer transfer. The reciprocating movement (straight line extension and retraction) is indicated by arrow 130, while the pivotal or rotational movement is indicated by arrow 140.
FIG. 2 illustrates a cross-sectional view of an exemplary semiconductor processing chamber, such as processing chamber 106a depicted in FIG. 1. Processing chamber 106a includes an inner chamber 202 for facilitating the flow of a process gas over the surface of a wafer. The housing includes a baseplate 204 having a gas inlet port 206 and a gas exhaust port 208. An upper clamp ring 210 and a lower clamp ring 212 act to hold a quartz cover member 214 and a quartz lower member 216 in place, respectively. Process gas is injected into chamber 202 through gas inlet port 206 which is connected to a gas source. Residual process gas and various waste products are continuously removed from the interior of chamber 202 through exhaust port 208. Arrows F indicate the typical flow path of a reactant gas passing through the chamber.
Wafers are placed into and removed from chamber 202 by the robotic wafer handling system 120 through an opening 203 formed in the side wall of the chamber.
A susceptor 224 holds the wafer in position during the semiconductor layer deposition process. As shown in FIG. 2, susceptor 224 includes a pocket 225 that is defined by at least one annular or planar bottom surface 226 and a cylindrical side wall 227. The depth of pocket 225 is generally chosen so that the top surface of the wafer being processed is approximately level with the top surface of the susceptor. Susceptor support 229 is coupled to susceptor 224 for rotating the wafer during the semiconductor fabrication process. Susceptor 224 also includes a plurality of through holes 240 for receiving at least three pins 242. Loading position pins 242 are attached to a support shaft 244 that provides vertical movement to raise and lower pins 242. Pins 242 are used to raise a wafer above susceptor surface 226 while the wafer is being loaded or unloaded into the chamber. Raising of the wafer prevents the robot blade from scraping or otherwise damaging the susceptor surface during the wafer loading or unloading procedure.
Heating lamps 228 and 230 provide infra-red radiant heat into the chamber through window portion 214 and quartz lower member 216 which are transparent to infra-red radiation.
In deposition processes, it is desirable to maximize wafer throughput while depositing film layers that have uniform thickness. With the increasing miniaturization of electronic circuits, there is a need to more accurately control the thickness of the deposition layers during semiconductor wafer processing. Among other requirements, in order to obtain uniform deposition layer thicknesses, it is important that the angular orientation of the wafer with that of the gas flow be essentially equal at all points along the wafer surface during the deposition process.
As discussed above, a robotic wafer handling system is often used to position a wafer within the pocket of a semiconductor processing chamber susceptor. As shown in FIGS. 3A and 3B, in some instances a wafer 300 is improperly placed on the susceptor 302. As a result, a portion of the wafer will reside outside of the susceptor pocket 304 causing the wafer to be out of alignment with the reactant gas flow stream. Currently, there is no method for detecting whether a wafer has been properly placed within the susceptor pocket.
The slant or tilt of the out-of-pocket wafer will result in an uneven film deposition across the surface of the wafer and a non-uniform resistivity. If the film deposition thickness or resistivity of a wafer is found to be non-uniform during post-process testing, that wafer and every wafer residing within the same cassette is discarded. This adversely affects throughput and results in higher processing costs.
Therefore, what is needed is a method and an apparatus for accurately determining the angular position of a wafer within a semiconductor processing chamber.